The German and European champion was not at its spectacular best but looked comfortable throughout, doing enough to earn a shot at lifting the trophy for a second time.
Bayern dominated the first half almost from start to finish but could only take a 1-0 lead into the break, Lewandowski getting his 28th goal of the season, and Al Ahly appeared encouraged after half-time.
However, the Egyptians did not have the quality to cause Bayern any sustained worries and Lewandowski sealed the win late on, with Hansi Flick taking his team to another final as he looks to make it six titles won in just 68 matches at the helm when they meet Tigres of Mexico in the showpiece.
It did not take Bayern long to establish control, as Serge Gnabry received Kinglsey Coman's cross and showed great awareness to calmly tee up Lewandowski for a simple finish in the 17th minute.
Further Bayern chances came thick and fast for the remainder of the first half, but the Germans were wasteful – Marc Roca shot wide and then also straight at Mohamed El-Shenawy in a two-minute spell, before Alphonso Davies dragged a right-footed effort past the left-hand post just before the break.
But Lewandowski was the surprising culprit a few moments prior to Davies' chance, as the Best FIFA Men's Player award winner lifted a chipped effort over the goal from inside the box after a clever release from Joshua Kimmich.
Ah Ahly looked a little more dangerous during the early stages of the second half, though Bayern were still unfortunate not to increase their lead when Thomas Muller agonisingly failed to reach David Alaba's across-goal header.
Bayern began to up the pressure again towards the end, with Ayman Ashraf almost slicing a cross into his own net, before Eric-Maxim Choupo-Moting headed at El-Shenawy and Corentin Tolisso shot just over from distance.
Although an equaliser looked unlikely, Bayern made sure of the win five minutes from time as Leroy Sane dug out a cross from the right and Lewandowski was on hand for the simplest of headers.